Conventionally, as disclosed in PTL 1 described below, a metal detector detecting a metal object has been known. Specifically, the metal detector includes a first coil, a second coil magnetically coupled with the first coil and configuring a resonant circuit in cooperation with a capacitor, and a control circuit having a function of detecting the object on the basis of a voltage between both ends of the resonant circuit. The first coil is supplied with an exciting current.
The metal detector configured as described above is slid along a surface of an object embedded with the metal object. When the metal object comes close to the first coil that makes up the metal detector by sliding the metal detector, an eddy current loss is caused by an electromagnetic induction to change an inductance of the first coil. When the inductance of the first coil changes, because an amplitude of a voltage between terminals of the resonant circuit is reduced, the metal object can be detected on the basis of the voltage between the terminals.
In the technique disclosed in the above PTL 1, the metal detector is required to be slid for the purpose of detecting the metal object. Thus, when a region in which the metal object is assumed to be present is large, there is a concern that the metal object cannot be appropriately detected such that man-hours for detecting the metal object increases.
As the metal object detection device applied to a non-contact power feeding device that performs power transfer between the primary coil and the secondary coil in a non-contact manner, as disclosed in the following PTL 2, a device having plural temperature sensors on the primary coil has been known. When the metal object is present on the primary coil at the time of non-contact power feeding, the device detects the metal object by detecting a rise in temperature of the metal object through a temperature sensor. The rise in temperature is caused when the eddy current flows in the metal object. According to the metal object detection device, when the metal object is detected, the power supply of the non-contact power feeding can be lowered, or the power supply can stop. As a result, a rise in the temperature of the metal object present on the primary coil can be suppressed, and further the safety of the non-contact power feeding device can be enhanced.
However, in the metal object detection device disclosed in the above PTL 1, there is a concern about disadvantages described below. In detail, the temperature of the metal object detected by the temperature sensor becomes lower than a real temperature of the metal object due to the presence of the metal object on the primary coil, or an air layer between the respective temperature sensors. This leads to a concern that a detection precision of the metal object is lowered. If the metal object is large, a time until the metal object becomes high in temperature since the non-contact power feeding starts is shortened. Thus, there is a concern that the metal object cannot be detected since the temperature of the metal object starts to rise due to the non-contact power feeding until the temperature of the metal object arrives at the temperature to be detected. Further, there is a concern that the metal object cannot be detected only during the operation of the non-contact power feeding device.